1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, and particularly to an apparatus for performing density control of images.
2. Related Background Art
Recently, image forming apparatus of an electrophotographic apparatus have come to be color type ones, and color image forming apparatus have adopted various methods such as an image on image development method, an intermediate transfer method and an electrostatic transfer member method.
In the image on image development method, toner images of a plurality of colors are superimposed and developed temporarily on a photosensitive drum and collectively transferred onto a transfer material to form a final image.
In the intermediate transfer method, toner images of a plurality of colors are superimposed and transferred temporarily onto an intermediate transfer member (primary transfer) and then collectively transferred onto a transfer material (secondary transfer) to form a final image.
In the electrostatic transfer member method, an electrostatic transfer member is made to attract a transfer material, and toner images are superimposed on the transfer material to form an image.
The above-described methods all have both advantages and disadvantages, that is, good points and bad points. Whichever method is adopted, there exists a defect that an original correct color cannot be obtained due to a little fluctuation of image density based on various conditions such as a change of the usage environment and the number of prints.
In the color image forming apparatus of the electrophotographic method, image density control is generally performed in which toner images for density detection (patches) are formed from respective color toners on trial, the toner amount is detected by a density sensor, and the result is fed back to image forming conditions. In general, in the image density control, maximum density control with the purpose of constantly maintaining the maximum density of each color is performed first, and then, halftone control with the purpose of constantly maintaining gradation characteristics of halftone to an image signal is performed.
FIG. 7 shows a density sensor 112 for measuring a toner adhering amount of a patch T, in which reference symbol 112a indicates a light emitting element such as an LED and reference symbol 112b indicates a light receiving element such as a photodiode. The patch T is generally formed on a base, that is, the photosensitive member in the image on image development method, the intermediate transfer member (for example, drum or belt) in the intermediate transfer method, or the electrostatic transfer member (for example, belt) in the electrostatic transfer member method.
When a measurement light L1 is irradiated from the light emitting element 112a, a light L2 reflected from a surface of the base of the patch T enters the light receiving element 112b, and a light receiving signal is output. In the case where the toner adhering amount of the patch T is large, the reflection light L2 is blocked much by the toner, and thus, a light receiving amount of the light receiving element 112b decreases. Conversely, in the case where the toner adhering amount of the patch T is small, the reflection light L2 is less blocked, and thus, the light receiving amount of the light receiving element 112b increases. In other words, in the case where the patch T completely covers the base, the light receiving signal is minimum while in the case where the patch T does not exist, the light receiving signal is maximum.
FIG. 8 shows a result of measurement of a black (K) toner with the above-mentioned density sensor. The horizontal axis indicates a toner adhering amount, and the vertical axis indicates a light receiving amount. The light receiving amount decreases as the toner adhering amount increases, and the toner adhering amount can be correctly measured.
FIG. 9 shows a result of measurement of a toner of color different from black with the above-mentioned density sensor. The horizontal axis indicates a toner adhering amount, and the vertical axis indicates a light receiving amount. At first, the light receiving amount decreases along with the increase in the toner adhering amount, but when the toner adhering amount reaches a constant amount (indicated by Q in the figure), the light receiving amount conversely increases. Thus, the toner adhering amount cannot be measured with accuracy.
This phenomenon is explained with reference to FIG. 10. The light L2 that enters the light receiving element 112b increases or decreases in accordance with the adhering amount of the toner on the base surface. However, in case of a color toner, a reflection light L3 that is a diffuse reflection light component caused by the toner portion exists, and the reflection light L3 also enters the light receiving element 112b. In case of a black toner, since the toner portion hardly attracts and reflects light, the reflection light L3 hardly exists even if the toner adhering amount increases. Thus, the result shown in FIG. 8 is provided. In case of the color toner, when the toner adhering amount increases, the reflection light L2 from the base surface decreases, but the reflection light L3 caused by the toner portion increases. Thus, the result shown in FIG. 9 is provided.
Further, as shown in FIG. 11, in the case where the light receiving element 112b is arranged in a normal direction to the patch T surface, if the base surface is near a mirror surface of the light receiving element 112b, the reflection light L2 from the base portion does not enter the light receiving element 112b, and the light receiving element 112b can receive and detect the diffuse reflection light L3 from the color toner portion. However, if the reflection light component from the base cannot be received, it becomes impossible to detect the black toner. In order to prevent this, it is sufficient that processing is performed such that the base surface is not made into a mirror surface to enable diffuse reflection. However, this again causes a problem in that the reflection component from the color toner portion and the reflection component from the base portion cannot be separated from each other at the time of detection of the color toner.
That is, in a conventional method, the reflection light L2 from the base and the reflection light L3 from the color toner portion cannot be reliably separated. Thus, detection accuracy is poor.
The present invention has been made in view of the above, and has an object thereof to provide an image forming apparatus in which a density for both a black toner and a color toner can be detected with accuracy while a structure thereof is not made complicated.
Another object of the present invention is to provide an image forming apparatus including:
image forming means for forming an image which is capable of forming a toner image for density detection; and
detecting means for irradiating light to the toner image for density detection and detecting light obtained from the toner image for density detection, in which:
the image forming means is controlled based on an output from the detecting means;
the toner image for density detection includes a first toner image and a second toner image having a light reflectance lower than that of the first toner image; and
the detecting means detects the first toner image formed on the second toner image in the case where a density of the first toner image is detected.
Still another object of the present invention is to provide an image forming apparatus including:
image forming means for forming an image which is capable of forming a toner image for density detection; and
detecting means for irradiating light to the toner image for density detection and detecting light obtained from the toner image for density detection, in which:
the image forming means is controlled based on an output from the detecting means;
the toner image for density detection includes a first toner image and a second toner image, the first toner image is a toner image of a color different from black, and the second toner image is a black toner image; and
the detecting means detects the first toner image formed on the second toner image in the case where a density of the first toner image is detected.
Other objects of the present invention will be apparent from the following description.